Synthesis of a new chitosan-p-tert-butylcalix[4]arene polymer as adsorbent for toxic mercury ion

In this paper, we have synthesized a novel chitosan-p-tert-butylcalix[4]arene polymer (CCP) as a highly efficient adsorbent for mercury ion (Hg2+) removal from water. In fact, a lower rim diamine derivative of p-tert-butylcalix[4]arene has been cross-linked with chitosan chain by carbonyl diimidazole (CDI) as the linker. CDI forms a urea linkage between calix[4]arene diamine derivative and amine groups of the chitosan polymeric chain. The structure and properties of the new polymer were characterized by Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscope. Also, the adsorption capacity of CCP was studied towards Hg2+ in aqueous medium by inductively coupled plasma-optical emission spectrometry. Interestingly, the results showed a considerable adsorption capacity for CCP in comparison with chitosan. Therefore, CCP can be introduced as a promising adsorbent for the elimination of Hg2+ from wastewaters. Moreover, because of the conformity of adsorption kinetic with pseudo-second-order kinetic models, it can be concluded that chemical adsorption has an important role between functional groups on CCP polymer and Hg2+ ions. In addition, according to Freundlich isotherm, the CCP surface was heterogeneous with different functional groups.

Comments to the Author(s) I would like to thank to the authors for their detailed research about this topic. This manuscript employs the adsorption of Hg2+ ions from water by using functionalized chitosan. The authors compared the adsorption capacity of newly synthesized functionalized-chitosan with the ones without functional group. Then, they characterized the structural properties through FTIR, XRD and SEM. The Hg+ adsorption capacity of synthesized material was investigated by ICP-MS. The research is quite comprehensive and includes enough physical characterization techniques for the synthesized polymers. Also, the authors studied the adsorption kinetics by Freundlich and Langmuir isotherms. According to the presented results, they reported that 25 mg of synthesized polymer could adsorb 94% of metal ions from 25 mL of 125 nmol/ml aqueous Hg2+ solution. The research contributes to the current literature about the utilization of functionalized-chitosan for metal ion removal applications. I would recommend that this manuscript is suitable for publication in 'Royal Society of Open Science' after the consideration of suggested points. You can find my comments and suggestions below. • The language of the manuscript should be reviewed. There are repetitive conjunctions in same sentences. • The authors should add the details about pH during the adsorption and they should include a pH study that shows the adsorption of Hg2+ vs pH of solution. • There are some other studies which use chitosan for the metal ion removal. They might be cited such as Biomacromolecules 2010, 11, 12, 3301-3308;RSC Adv., 2013,3, 7828-7837. • In Figure 1, the peaks might be labelled to show the functional groups. • In section 3.5, the paragraph starts with 'SEM also known as SEM microscopy'. I think, it's not a true definition because SEM abbreviation already includes 'microscopy' term. • In Figure 3 and 4, there are (a), (b), (c) sections that identify the magnification of micrographs. The authors gave the scale bar dimensions in the caption but is seems like the size of chitosan. If they can give the magnification values, the caption will be more clear. • In Figure 6, the standard deviation of chitosan was given horizontally. Is it true? I think standard deviations give the deviation from average adsorption (qe) value but it seems like the deviation from time. • In Section 3.9, the last sentence says that 'This means there are two kind of adsorption process between different functional groups of the CCP and Hg+2.' What do you mean here? Why does it depend on the concentration? Could you give more details about it? • The results are given as % sorption by using X mg of polymer from X mL of X mol solution. I think it is pretty confusing. The discussion about the adsorption results should be done as mg (ion)

Decision letter (RSOS-211223.R0)
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Dear Dr Zadmard: Title: Synthesis of a new chitosan-p-tert-butylcalix[4]arene polymer as adsorbent for toxic Hg2+ metal ion Manuscript ID: RSOS-211223 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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RSC Subject Editor
Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) Comments The research article "Synthesis of a new chitosan-p-tert-butylcalix[4]arene polymer as adsorbent for toxic Hg2+ metal ion" seems to be a good work by the researchers, however it requires a major revision so as to be published in this reputed journal. 1. The introduction section needs improvement in terms of language and content. The authors should consult native English speaker and rewrite the introduction and rewrite the introduction with the updated references. a. RSC Advances 2019, 10 (5), <a href="tel:2943-2943">2943-2943</a> b. ACS omega 2019, 4 (17), 17425-17437 c. International journal of biological macromolecules, 2019, 136, 189-198 2. The authors have focussed mainly on synthesis and characterization of the product, however the adsorption properties which is the application part needs attention. 3. Effect of pH on the adsorption sites and the adsorption of Hg2+ on chitosan-p-tertbutylcalix[4]arene polymer must be explained with point of zero charge of the adsorbent. 4. The selective adsorption of Hg2+ on chitosan-p-tert-butylcalix[4]arene polymer must be justified by adsorption kinetics and thermodynamic data. Furthermore, the spontaneity of the adsorption of Hg2+ on chitosan-p-tert-butylcalix[4]arene polymer must be explained using Van't Hoff plots 5. Error bars must be incorporate with all the data plots, to justify the selective adsorption of of Hg2+ on chitosan-p-tert-butylcalix[4]arene polymer. 6. Mere calculation of adsorption data from the kinetic and isotherm plots is not enough for conclusion of the adsorption phenomena. The validation of the results must be ensured by statistical error analysis by the sum of the square of the error (SSE), sum of absolute error (SAE), chi square (ᵡ2), and standard deviation Δq (%), between the experimental data and calculated values.
Reviewer: 2 Comments to the Author(s) I would like to thank to the authors for their detailed research about this topic. This manuscript employs the adsorption of Hg2+ ions from water by using functionalized chitosan. The authors compared the adsorption capacity of newly synthesized functionalized-chitosan with the ones without functional group. Then, they characterized the structural properties through FTIR, XRD and SEM. The Hg+ adsorption capacity of synthesized material was investigated by ICP-MS. The research is quite comprehensive and includes enough physical characterization techniques for the synthesized polymers. Also, the authors studied the adsorption kinetics by Freundlich and Langmuir isotherms. According to the presented results, they reported that 25 mg of synthesized polymer could adsorb 94% of metal ions from 25 mL of 125 nmol/ml aqueous Hg2+ solution. The research contributes to the current literature about the utilization of functionalized-chitosan for metal ion removal applications. I would recommend that this manuscript is suitable for publication in 'Royal Society of Open Science' after the consideration of suggested points. You can find my comments and suggestions below. • The language of the manuscript should be reviewed. There are repetitive conjunctions in same sentences.
• The authors should add the details about pH during the adsorption and they should include a pH study that shows the adsorption of Hg2+ vs pH of solution.
• There are some other studies which use chitosan for the metal ion removal. They might be cited such as Biomacromolecules 2010, 11, 12, <a href="tel:3301-3308">3301-3308</a>; RSC Adv., 2013,3, <a href="tel:7828-7837">7828-7837</a>. • In Figure 1, the peaks might be labelled to show the functional groups. • In section 3.5, the paragraph starts with 'SEM also known as SEM microscopy'. I think, it's not a true definition because SEM abbreviation already includes 'microscopy' term. • In Figure 3 and 4, there are (a), (b), (c) sections that identify the magnification of micrographs. The authors gave the scale bar dimensions in the caption but is seems like the size of chitosan. If they can give the magnification values, the caption will be more clear. • In Figure 6, the standard deviation of chitosan was given horizontally. Is it true? I think standard deviations give the deviation from average adsorption (qe) value but it seems like the deviation from time. • In Section 3.9, the last sentence says that 'This means there are two kind of adsorption process between different functional groups of the CCP and Hg+2.' What do you mean here? Why does it depend on the concentration? Could you give more details about it?
• The results are given as % sorption by using X mg of polymer from X mL of X mol solution. I think it is pretty confusing. The discussion about the adsorption results should be done as mg (ion) per g (polymer) (mg/g) -the units may change. It makes the result comparable with the current literature.

Recommendation? Accept as is
Comments to the Author(s) I would like to thank to the authors for their clear and point by point response to the reviewer's comments. I would recommend that this manuscript is suitable for publication in 'Royal Society of Open Science'.

Decision letter (RSOS-211223.R1)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear Dr zadmard:
Title: Synthesis of a new chitosan-p-tert-butylcalix[4]arene polymer as adsorbent for toxic Hg2+ metal ion Manuscript ID: RSOS-211223.R1 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.
Please see the Royal Society Publishing guidance on how you may share your accepted author manuscript at https://royalsociety.org/journals/ethics-policies/media-embargo/. After publication, some additional ways to effectively promote your article can also be found here https://royalsociety.org/blog/2020/07/promoting-your-latest-paper-and-tracking-yourresults/.
Thank you for your fine contribution. On behalf of the Editors of Royal Society Open Science and the Royal Society of Chemistry, I look forward to your continued contributions to the Journal. According to the respected reviewer, these above mentioned 3 references (Ref. No. [34], [37] and [38]) were added to the introduction and the text was rewritten.
2. The authors have focused mainly on synthesis and characterization of the product, however the adsorption properties which is the application part needs attention.

Response:
The main purpose of this study was to synthesize and identify a new polymer based on calix[4]aren. Although we have provided some of the absorption properties of this polymer, our team is conducting additional studies for the application of this new polymer, the results of which will be presented in future publications.
3. Effect of pH on the adsorption sites and the adsorption of Hg 2+ on chitosan-p-tertbutylcalix[4]arene polymer must be explained with point of zero charge of the adsorbent.

Response:
In this article, all studies have been done in comparison with chitosan polymer and chitosan is sensitive to pH and dissolves at acidic pH. Therefore, the adsorption process was studied at neutral pH (please see the reference 55). In addition, in some similar studies of our work, chitosan has been used as an adsorbent at a specific pH without investigating the effect of different pH (please see the reference 22). 4. The selective adsorption of Hg 2+ on chitosan-p-tert-butylcalix[4]arene polymer must be justified by adsorption kinetics and thermodynamic data. Furthermore, the spontaneity of the adsorption of Hg 2+ on chitosan-p-tert-butylcalix[4]arene polymer must be explained using Van't Hoff plots.

Response:
Thanks for your thoughtful comment. In this study, the adsorption process was investigated at ambient temperature. Therefore, it is not possible to study the absorption process using van't Hoff equation. There is also no claim that mercury absorption is specific in this study, and it is possible that other metal cations also have interactions with this adsorbent, which will be investigated in a follow-up study.
5. Error bars must be incorporate with all the data plots, to justify the selective adsorption of Hg 2+ on chitosan-p-tert-butylcalix[4]arene polymer.

Response:
It was corrected according to your comment.
6. Mere calculation of adsorption data from the kinetic and isotherm plots is not enough for conclusion of the adsorption phenomena. The validation of the results must be ensured by statistical error analysis by the sum of the square of the error (SSE), sum of absolute error (SAE), chi square (ᵡ2), and standard deviation Δq (%), between the experimental data and calculated values.

Response:
This is a reasonable suggestion which is very much appreciated. We revised accordingly to include this information. We calculated all requested values and summarized in the

Response to Reviewer 2 Comments:
The language of the manuscript should be reviewed. There are repetitive conjunctions in same sentences. Response: The text was rewritten and the corrections were made according to your comment as much as possible.
The authors should add the details about pH during the adsorption and they should include a pH study that shows the adsorption of Hg 2+ vs pH of solution. Response: Thanks for your comment. As was mentioned in response to reviewer 1 (comment 3), In this article, all studies have been done in comparison with chitosan polymer which is sensitive to pH and dissolves at acidic pH. Therefore, the adsorption process was studied at neutral pH (please see the reference 55). In addition, in some similar studies of our work, chitosan has been used as an adsorbent at a specific pH without investigating the effect of different pH (please see the reference 22).
In Figure 1, the peaks might be labelled to show the functional groups.
Response: Figure 1 was modified according to your suggestion. (in the manuscript, page 9) In section 3.5, the paragraph starts with 'SEM also known as SEM microscopy'. I think, it's not a true definition because SEM abbreviation already includes 'microscopy' term.

Response:
It was corrected according to your comment. (in the manuscript, page 11) In Figure 3 and 4, there are (a), (b), (c) sections that identify the magnification of micrographs. The authors gave the scale bar dimensions in the caption but is seems like the size of chitosan. If they can give the magnification values, the caption will be more clear.

Response:
Figures 3 and 4 were modified according to your comment. (in the manuscript, page 11) In Figure 6, the standard deviation of chitosan was given horizontally. Is it true? I think standard deviations give the deviation from average adsorption (qe) value but it seems like the deviation from time.
Response: Figure 6 was corrected according to your comment. (in the manuscript, page 13) In Section 3.9, the last sentence says that 'This means there are two kind of adsorption process between different functional groups of the CCP and Hg +2 .' What do you mean here?
Why does it depend on the concentration? Could you give more details about it? Response: Since the polymer adsorbent consists of different functional groups as hydroxyl group, amid bond, aromatic rings for π interactions and etc., here (in the manuscript, section 3.9, page 15) the authors aim to show that, according to the diagram, different interactions are involved in the adsorption process but apparently misleads the reader. Therefore, this sentence was deleted.
The results are given as % sorption by using X mg of polymer from X mL of X mol solution.
I think it is pretty confusing. The discussion about the adsorption results should be done as mg (ion) per g (polymer) (mg/g)the units may change. It makes the result comparable with the current literature.

Response:
Thank you for your accurate comment. Your opinion is quite correct. Most studies in this field report the adsorption capacity in (mg/g), but there are a number of articles that report the adsorption capacity as a percentage of adsorption, from which we took a model. A comparison table and relevant references were added in Section 3.6 (in the manuscript, page 12), and the confusing sections were rewritten.
In the last part, there is a need for discussion about the results. The literature studies about the Hg +2 removal by using chitosan should be compared with this research's results. Why these polymers are promising and why people should prefer this method? Response: A comparison table was added in Section 3.6 of the text (page 12). The high efficiency in the synthesis and powderiness of this new compound improves its use as an adsorbent. Also, the resistance of this polymer in acidic environments and its insolubility in a wide range of pH are other advantages of this new synthetic polymer.
What about the performance of polymers in the presence of competitive double valent ions?
Do the authors have any result about selectivity of polymer? Response: The main purpose of this study was to focus on the synthesis and identification of a new polymer, and currently, our team is conducting additional studies for the application of this new polymer, including the issues of selectivity and competitive adsorption in the presence of other ions.
What is the reusability property of polymers? Can we regenerate them and how is the adsorption / desorption cycle? Response: Thank you for your comments. The desorption process was not studied in this work. A search of the literature shows that there are many articles that, like our work, have not studied desorption despite studying the absorption process (please see the references 21, 22 and 55).